The long-term objective of this ongoing project remains the development of appropriate statistical techniques for the analysis of data arising from assays under study in the Environmental Mutagen Test Development Program (EMTDP). Results are also applicable to other large mutagenicity studies, such as the European Collaborative Study and the International Program for the Evaluation of Short-Term Tests for Carcinogenicity. The major focus to date has been on microbial test systems, although additional research has dealt with Drosophila. Preliminary work on in vivo and in vitro cytogenetic assays has also commenced. Statistical procedures for the design and analysis of short-term tests proposed or currently employed by other researchers in mutagenicity are assessed and new and improved procedures are developed.